1. Field of the Invention
The present invention relates to a powdered photocatalyst and its manufacturing method and, more particularly, to nanopowders of the ZnO photocatalyst activated by UV or visible light and its manufacturing method.
2. Description of Related Art
Photocatalysts such as ZnO and TiO2 nano-particles have drawn much attention due to their applications in antibacterials, water treatment, deodorants, NOX decomposition, self-cleaning and so on. Conduction-band electrons and valence-band holes are generated on its surface when a photocatalyst is illuminated by light with energy greater than its band gap energy. Holes can then react with water molecules adhering to the surface of the photocatalyst to form highly reactive hydroxyl radicals (OH.). Oxygen here acts as an electron acceptor by forming a super-oxide radical anion (O2−.) on the surface. The super-oxide radical anions may act as oxidizing agents or as an additional source of hydroxyl radicals via the subsequent formation of hydrogen peroxide. The powerful oxidants associated with hydroxyl radicals are able to oxidize organic materials. When the cell membrane of bacteria is in contact with the powerful oxidants, it will be decomposed and consequently the bacteria will die. The body of the bacteria will eventually be decomposed into carbon dioxide and water. Photocatalysts not only kill the bacteria, but also clean their bodies. Therefore, the objective for cleaning and sterilization can be easily achieved by the assistance of photocatalyst.
In addition, the exposure of the photocatalyst to the light radiation can also increase the hydrophility of the photocatalyst by forcing the adsorbed water to penetrate into the interfaces between the pollutant and the photocatalyst. Owing to the increased hydrophile, the pollutants adhering to the surface can be easily removed through washing. Therefore, the self-cleaning advantage of the photocatalyst can be applied to, for example, streetlamp covers and outdoor walls/windows, for keeping their cleanliness for a long time.
Because of its band-gap energy of 3.2 eV, ZnO absorbs UV light with the wavelength equal to or less than 385 nm. It is known that visible light (wavelength between 400 and 700 nm) accounts for 45% of energy in the solar radiation while in UV light it is less than 10%. Even in the radiation of the cold cathode fluorescence lamp, illuminance of only 0.1 mW/cm2 is in the near-UV band. Since the illuminance is reversely proportion to the square of the distance from the light source, the illuminance for most objects in a room is only around 0.1 μW/cm2. Under regular indoor illuminance, it is very hard to activate the photocatalyst for reaction effectively. Therefore, as far as photocatalytic efficiency or indoor applications are concerned, it is desirable that photocatalyst such as ZnO can absorb not only UV but also visible light.
Since the high surface area and the special structure of nanopowders (with an average diameter less than 100 nm), they have special magnetic, dielectric, optical, and thermal properties. For increasing the surface area and thus the effective reaction area of a photocatalyst, most manufacturing make considerable efforts to reduce the size of the particles. For conventional photocatalyst powders (with an average particle size greater than 100 nm), the electrons and holes will be relatively easy to recombine during the transportation in the particles. Moreover, the surface area of conventional photocatalysts is low. These two effects reduce the activity of photocatalysis. Generally, the size of nanoparticles (or nanopowders) greatly depends on the manufacturing methods. Furthermore, the material itself and its structure also determine the properties of the photocatalysts. Therefore, the method for manufacturing a photocatalyst in a nanometer scale for visible light is the key factor for commercial applications in the future.
In the past time, a sputtering method for manufacturing photocatalysts for visible light was found in the disclosure of JP 2001-205094. The photocatalyst film is made by sputtering a target of SnO2 and ZnO to a substrate in an atmosphere of nitrogen in the cited patent. Another patent, JP 11-290697 disclosed a method for manufacturing visible light TiO2 photocatalyst through sol-gels doped with transition metal elements. Since further purification for removing contaminated cations from the mixtures of the starting materials such as metal alkoxide and metal inorganic salt is required, the microstructure, physical properties, and chemical properties of the final product are various.
Another method for manufacturing photocatalyst for visible light is disclosed in JP 09-192496, wherein a photocatalyst is made up of at least one oxide selected from titanium dioxide, zinc oxide, and tungsten oxide, SrTiO3, or SiC, and doped with at least one transition metal element selected from the group consisting of V, Cr, Mg, Fe, Co, Ni, and Cu. The amount of these dopants is controlled in the range from 500 ppb to 500 ppm. The JP 09-262482 reports implanting at least one element into the surface of TiO2 makes a photocatalyst for visible light. The elements for doping are selected from a group consisting of Cr, V, Cu, Fe, Mg, Ag, Pd, Ni, Mn and Pt. The concentration of the implanted ion is ≧1*1015 ions/g and the accelerating voltage is ≧30 Kev. In addition to implanting metal elements to the surface of TiO2, methods for implanting hydrogen or alkaline metal ions to the surface (e.g. JP 2000-103647) or for treating in hydrogen plasma or plasma of other rare gases are also reported (e.g. JP 2001-212457).
In addition, a method for forming photocatalyst TiC on the surface of TiO2 by treating TiO2 in the plasma of a mixture of gaseous methane and hydrogen is reported in U.S. Pat. No. 6,306,343.